The present invention relates to a development method and apparatus for use in image formation apparatus such as printers, facsimiles and copiers, and also relates to an image formation apparatus and a process cartridge. More particularly, it relates to a development method of developing an electrostatic latent image formed on an image carrier using a developer held on a developer carrier to form a toner image on the image carrier at a development region (development nip region) between the developer carrier and the image carrier opposing thereto. The present invention further relates to a development apparatus for executing the development method, an image formation apparatus using the development apparatus and a process cartridge including the development apparatus mounted thereon.
Conventionally known development systems for developing the electrostatic latent image on the image carrier in the development nip region are classified into a two-component development type and a one-component development type. The former employs a two-component developer that contains toner and magnetic particles to develop the electrostatic latent image. The latter employs a one-component developer consisting only of toner to develop the electrostatic latent image.
The two-component development system holds the two-component developer consisting of toner and magnetic particles in the form of a brush to form a magnetic brush on the surface of a developer carrier that contains magnets inside. When only toner from the magnetic brush is attached to the electrostatic latent image formed on the image carrier at the development region (development nip region) between the developer carrier and the image carrier opposed thereto, a toner image is formed on the image carrier. This two-component development system is known as an excellent development system because it has a stable and nice transfer property of toner images and development property against temperature and humidity.
In the one-component development system, on the other hand, the one-component developer consisting only of toner is held in the form of a uniform thin film on the surface of the developer carrier. The toner held on the surface of the developer carrier is opposed to an electrostatic latent image formed on the image carrier in a contact or non-contact state. When the toner attaches to the electrostatic latent image at the development region (development nip region) between the developer carrier and the image carrier opposing thereto, a toner image is formed an the image carrier. This one-component development system is known as an advantageous development system because it can provide an image with a high image quality and high resolution. The one-component development system does not employ the magnetic particles like the developer in the two-component development system and accordingly has no disadvantage to cause a development variation due to the magnetic brush that contacts with the surface of the image carrier. Therefore, it is possible to perform development that is highly faithful to the electrostatic latent image.
A development system that has advantages of the one- and two-component development types in combination has been proposed as a development apparatus that employs a developer carrier serving as a toner supplier member having a magnetic brush composed of a two-component developer formed on the surface. In the development apparatus, the magnetic brush on the toner supplier member is employed to supply a one-component developer composed only of toner to the developer carrier serving as a toner carrier (see Japanese Patent Application Laid-Open Nos. 56-40862 and 59-172662, for example).
In these development apparatus, the two-component developer is agitated in the development apparatus to hold the two-component developer on the toner supplier member (such as a magnetic roller and a magnetic brush forming member) to form a magnetic brush. The toner in the magnetic brush is charged to have a certain polarity through friction with magnetic particles. From the magnetic brush on the toner supplier member, only the toner charged to have a certain polarity is displaced to and held on the toner carrier (such as a development roller and a toner layer holding member).
This development apparatus can reduce the stress that is imparted on the toner carrier and the toner more effectively compared with the one-component development system. In addition, as it performs development in the one-component development system, a high image-quality same as that in the one-component development system can be achieved.
In any development systems, however, the above development apparatus have the following subject matters.
After the electrostatic latent image on the image carrier is developed (visualized), the developer layer on the developer carrier is divided into a toner-consumed layer and a toner-resided layer. In the toner-consumed layer, toner corresponding to the image section in the electrostatic latent image is consumed by the development. In the toner-resided layer, toner corresponding to the non-image section in the electrostatic latent image is resided, not consumed.
In such the development apparatus, a difference between the toner-consumed layer and the toner-resided layer occurs in an amount of toner held on the surface of the developer carrier. To resolve the difference in the amount of toner held on the developer carrier, it is required to supply a new toner to the toner-consumed layer through rotations of the developer carrier by an amount corresponding to the consumed amount. It is difficult in practice, however, to supply the new toner only to the toner-consumed layer on the surface of the rotating developer carrier. Accordingly, it is not possible to resolve the difference in the amount of toner held on the developer carrier using the method of simply supplying the new toner. As a result, such the development apparatus causes a hysteresis. For example, due to the difference in the amount of toner held on the developer carrier, an afterimage (ghost) appears as a density difference in an image at the time of a subsequent development.
In order to resolve such the hysteresis, it is effective to once remove the developer from a location in the surface of the developer carrier after developing (visualizing) the electrostatic latent image on the image carrier. A new developer is then supplied to the location in the surface of the developer to resolve the above-described difference in the amount of toner held on the surface of the developer carrier.
To execute such the method of removing and supplying the developer, the development apparatus requires a developer replacement unit which replaces the developer held on the developer carrier. The developer replacement unit may include a blade, scraper or brush roller as a unit which contacts with the surface of the developer carrier to mechanically remove the whole developer from the developer carrier. If such the unit is employed, however, the abrasion over time of the surface of the developer carrier changes the properties of carrying and holding the developer and invites a malfunction because toner spent is caused from the stress during removal. Also in this case, it is required to ensure a space for installing the blade, scraper or brush roller around the developer carrier, resulting in a larger body of the development apparatus.
An approach for resolving such the malfunction due to the use of the mechanical developer replacement unit employs an arrangement of magnetic poles of magnets arranged inside the developer carrier to remove the developer on the developer carrier magnetically. This approach can not be applied to the one-component development apparatus that employs non-magnetic toner, however, although it is effective in the two-component development apparatus and the one-component development apparatus that employs magnetic toner.
Japanese Patent Application Laid-Open No. 6-67546 proposes a development apparatus that can remove the developer (toner) from the developer carrier without the use of the above mechanical developer replacement unit and the magnets. This development apparatus has advantages of the one- and two-component development types in combination. It includes an electric field switcher unit which switches an electric field located across the developer carrier and the toner carrier to shift toner intermittently from the toner carrier to the developer carrier. In this development apparatus, the electric field switcher unit is employed to remove toner from the toner carrier without imparting a large mechanical stress on the toner and the surface of the toner carrier for holding the toner. In addition, it is possible to provide a highly reliable development apparatus that does not cause the above hysteresis. The development apparatus has disadvantages, however, because of complications in configurations of the apparatus itself and the electric field switcher unit as well as increased costs.
It is generally considered that the above hysteresis is caused from a difference in the amount of toner held on the surface of the developer carrier between the toner-consumed layer and the toner-resided layer held on the surface of the developer carrier after development.
In the conventional development apparatus, even if the amount of toner held on the surface of the developer carrier is an amount neither short nor over but sufficient to develop the electrostatic latent image, an afterimage maybe caused supposedly due to the hysteresis. For example, as shown in FIG. 3A, a draft image having a black solid section A at the top left, a non-image section (or white image section) B at the top light and a low contrast section C at the rear can be employed to forman image. In this case, an afterimage may be formed while a sufficient amount of toner is supplied to the surface of the developer carrier. In a normal print image formed using the draft image, as shown in FIG. 3B, a low contrast print image c corresponding to the low contrast section C is expected to have a uniform image density. In a practical print image, however, a low contrast print image corresponding to the low contrast section C may include a low contrast print image c2 at a part immediately beneath a blank image b corresponding to the non-image section B as shown in FIG. 3C. This low contrast print image c2 has an image density darker than that of a low contrast print image c1 at a part immediately beneath a black solid image a corresponding to the black solid print image section A as a phenomenon (hereinafter referred to as an xe2x80x9cafterimage phenomenonxe2x80x9d).
The Inventors have researched through various experiments to study the cause of such the phenomenon and consequently found the following fact. In the case of the draft image having the non-image section B as above, in the electrostatic latent image formed on the image carrier, a part corresponding to the non-image section B has a potential equal to the ground potential on the image carrier. Therefore, when the toner held on the developer carrier which forms the low contrast print image c2 opposes to the part corresponding to the non-image section B on the surface of the image carrier, a charge is injected into the toner from the developer carrier that holds the toner. Because the surface potential on the image carrier corresponding to the non-image section B has a large potential difference from the development bias that has the opposite polarity and is applied to the developer carrier.
This potential difference reduces an amount of charge (q/m) on toner held on the developer carrier which forms the low contrast print image c2. This reduction in the amount of charge on toner lowers a force of the toner for attaching to the surface of the developer carrier. Accordingly, a much lager amount of toner tends to attach to the low contrast print image c2 at the part immediately beneath the blank image b corresponding to the non-image section B. As a result, the low contrast print image c2 comes to have an image density darker than that of the low contrast print image c1 at the part immediately beneath the black solid image a corresponding to the black solid print image section A.
It is an object of this invention to provide a development method capable of forming a uniform dense image by suppressing a variation in an amount of charge (q/m) on toner held on a developer carrier to reduce an afterimage phenomenon. An image formation apparatus that employs the development apparatus and a process cartridge that includes the development apparatus mounted thereon are also provided.
According to one aspect of the present invention, the amount of charge q/m on toner held on the surface of the developer carrier comes to have a variation rate of 45% or below. Such the variation rate can lower an afterimage rate R to 2% or below. This is effective to suppress the above-described afterimage phenomenon and give a toner density variation within tolerance to the toner image. The variation rate of the amount of charge q/m on toner is defined as a variation rate between charges before and after the charge injection into the toner. The afterimage rate R is defined as a ratio calculated on the basis of an optical reflectivity ID2 at the low contrast print image c2 and an optical reflectivity ID1 at the low contrast print image c1, using the following equation, R={(ID1xe2x88x92ID2)/(ID1+ID2)}xc3x97100%.
According to another aspect of the present invention, the amount of charge q/m on toner held on the surface of the developer carrier comes to have a variation rate of 45% or below. It is effective to form a uniform dense toner image on the image carrier with the absolute value of an afterimage rate |R| within 2% and the suppressed afterimage phenomenon.
According to still another aspect of the present invention, the toner layer held on the surface of the developer carrier contacts with the conductive member. This allows the charge injected from the image carrier to the toner to be dissipated through the conductive member and the amount of charge on toner once reduced by the charge injection to be restored to the previous charge amount before reduction. This is effective to form a uniform dense toner image on the image carrier with the suppressed afterimage phenomenon.
According to still another aspect of the present invention, the development apparatus according to the above-mentioned aspects is employed as a development unit in the image formation apparatus.
According to still another aspect of the present invention, the development apparatus according to the above-mentioned aspects is employed as a development unit in the process cartridge for use in an image formation apparatus.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.